1. Field of the Invention
The present invention is in the field of measuring physiological function of a group of body cells.
2. Description of the Background Art
Current clinical practice for determining liver function includes deriving a CTC score, which is a compilation of laboratory data and clinical assessment of ascites and encephalopathy. D. A. Noe and R. C. Rock (eds), Laboratory Medicine, The Selection and Interpretation of clinical Laboratory Studies, Williams and Wilkins, 1994, Baltimore, Md., Chapter 5, Assessment of Organ Function, by D. A. Noe, p. 55-60, Chapter 19, Liver and Biliary Tract, by A. T. Blei, p. 363-379, Chapter 21, The Kidneys, by D. A. Oken and A. C. Schoolwerth, p. 401-410.
Another test involves the use of indocyanine green (ICG). ICG is known to be exclusively cleared from the bloodstream by the liver. Thus a measurement of the ICG blood clearance time profile is directly related to liver function. J. Caesar, S. Shaldon, L. Chiandussi, L. Guevara, and Sheila Sherlock, "The use of indocyanine green in the measurement of hepatic blood flow and as a test of hepatic function," Clin. Sci. 21, 43-57 (1961).
The ICG test has undergone an evolution in technology. In its first incarnation, blood was withdrawn from the subject at several times following an IV bolus injection. The blood samples were then processed spectrophotometrically to determine ICG concentration. R. Jalan and P. C. Hayes, "Review article: quantitative tests of liver function," Aliment Pharmacol. Ther. 9, 263-270 (1995); A. W. Hemming, C. H. Scudamore, C. R. Shackleton, M. Pudek, and S. R. Erb, "Indocyanine green clearance as a predictor of successful hepatic resection in cirrhotic patients," Am. J. Surg. 163, 515-518 (1992); P. Ott, S. Keiding, and L. Bass, "Plasma elimination of indocyanine green in the intact pig after bolus injection and during constant infusion: comparison of spectrophotometry and high-pressure liquid chromatography for concentration analysis," Hepatology 18, 1504-1515 (1993). Subsequently, a non-invasive technique employing ear densitometry was developed. C. M. Leevy, F. Smith, J. Longueville, G. Paumgartner, and M. M. Howard, "Indocyanine green clearance as a test for hepatic function: Evaluation by dichromatic ear densitometry," Journal of Medicine 24, 10-27 (1993). Problems associated with the clinical development of this device recently led Japanese researchers to improve upon the ear densitometry technique. This newer method, termed the finger-piece method, employs transmitted light of two wavelengths measured throughout a finger to deduce ICG concentration. M. Kanda, K. Taniguchi, K. Awazu, Y. Ishigami, M. Masuzawa, and H. Abe, "Continuous monitoring of Cardiogreen removal by a diseased liver using an optical sensor," Proc. SPIE 904, 39-46 (1988); M. Nakayama, N. Kanaya, S. Fujita, and A. Namiki, "Effects of ephedrine on indocyanine green clearance during spinal anesthesia: Evaluation by the finger piece method," Anesth. Analg. 77, 947-949 (1993); N. Kanaya, H. Iwasaki, and A. Namiki, "Noninvasive ICG clearance test for estimating hepatic blood flow during halothane and isoflurane anaesthesia," Can. J. Anaesth. 42, 209-212 (1995); N. Kanaya, M. Nakayama, S. Fujita, and A. Namiki, "Comparison of the effects of sevoflurane, isoflurane and halothane on indocyanine green clearance," Br. J. Anaesth. 74, 164-167 (1995); S. Shimizu, W. Kamiike, N. Hatanaka, Y. Yoshida, K. Tagawa, M. Miyata, and H. Matsuda, "New method for measuring ICG R.sub.max with a clearance meter," World J. Surg. 19, 113-118 (1995).
Both ear densitometry and the finger-piece method involve measuring absorption (or transmission) of light by ICG.
Also of interest is that in vitro fluorometric determination of ICG in plasma has been demonstrated, B. Hollins, B. Noe, and J. M. Henderson, "Fluorometric determination of indocyanine green in plasma," Clin. Chem. 33, 765-768 (1987).
Other references of general interest include: R. L. Sheridan, et al., "Burn depth estimation by indocyanine green fluorescence: Initial human trial," Journal of Burn Care & Rehabilitation 16, 602-604 (1995); M. A. O'Leary, D. A. Boas, B. Chance, and A. G. Yodh, "Reradiation and imaging of diffuse photon density waves using fluorescent inhomogeneities," Journal of Luminescence 60 & 61, 281-286 (1994); X. Li, B. Beauvoit, R. White, S. Nioka, B. Chance, and A. Yodh, "Tumor localization using fluorescence of indocyanine green (ICG) in rat models," Proc. SPIE 2389, 789-797 (1995).
There remains a need in the art for improved methods of measuring physiological function.